This invention relates to a swivel system particularly suited to the mooring of a floating facility to a marine structure with a virtually fixed position relative to the seabed and to which the floating facility is secured through a rigid arm.
The floating facility may be a ship and the marine structure a mooring buoy anchored at a distance from shore. The marine structure may also be a tower, a dolphin, an articulated leg, a buoyant column, a mooring trunk, a tank, etc. It may also be a buoy for loading and/or unloading ship cargo, a facility from which operations such as seabed drilling, oil recovery, etc. are carried out or controlled.
The floating facility may also be a plant for the recovery of thermal energy from seas or a floating oil processing and storage plant.
In most cases, the swivel system has to transmit important forces while allowing relative free movement between the arm of the floating facility and the marine structure. Such freedom of motion is required to allow for the dynamic attack by wind, wave and currents.
Wave action requires firstly free nutation movement about a vertical axis to cater for the floating facility roll, pitch and heave and secondly free swaying movement in a horizontal plane for small alternating motions. Wind and current direction variations require that the floating facility be able to weathervane about the marine structure which is in a fixed position relative to the seabed, the swivel joint allowing a rotation of more than one full turn.
Swivel systems are already in use for ship mooring to structures such as trunk buoys or articulated legs which provide the above defined freedom of motion. These systems are based on ball or roller bearings and combine a guide ring or turntable with a vertical axis and a universal coupling pivoting about two perpendicular horizontal axes.
Other systems with the same geometry are also in use which are based on journal bearings with lubricated copper alloy bushings.
Such systems are little suited to the particular stresses imposed by marine environment on such facilities.
Ball and roller bearings are designed for continuous running and their operation is upset by the slightest corrosion.
Under offshore conditions, movements are slow and with small amplitudes and fatigue stresses are constantly applied to the same area of the bearing which is only subjected to small alternating motions with an amplitude less than 10.degree.. Also, corrosion due to marine environment is severe.
Journal bearings with copper alloy bushings are generally subject to rapid wear thus increasing running play which in turn prompts wear due to the smaller area which is in contact with the shaft. Frequent replacements are required. Maintenance operations at sea are difficult and cost intensive, especially when, as this is the case with oil recovery facilities, they involve oil production disruption.
The prior art may be illustrated by the following french patent FR-A No. 2 171 478 and the following U.S. Pat. Nos. 3,614,869, 4,262,380, 4,494,475, 4,439,055 and 4,435,097.